The present invention relates to a method of controlling the temperature of a fixing device incorporated in an electrophotographic copier, printer or similar image forming equipment.
A fixing device for the above application has a heat roller accommodating a heater therein, a press roller held in pressing contact with the heat roller, and a thermistor responsive to the surface temperature of the heat roller. The temperature of the heater is controlled on the basis of a signal representative of a temperature sensed by the thermistor. The thermistor may be held in contact with the surface of the heat roller, as taught in, for example, Japanese Patent Laid-Open Publication No. 49174/1987, or may be spaced apart from it, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 88170/1986. The problem with the thermistor contacting the heat roller is that the former is apt to scratch or otherwise damage the latter due to friction. While this problem may be eliminated if the thermistor contacts part of the heat roller other than the part which a paper sheet passes, the thermistor in such a position cannot measure the temperature of the part which a paper sheet passes. This is critical considering the fact that the temperature of the part of the heat roller which a paper sheet passes drops on the passage of a paper sheet. On the other hand, when the thermistor is spaced apart from the heat roller, the roller is free from damage. With this configuration, however, it is necessary that the gap between the heat roller and the thermistor be supervised with extreme accuracy to accurately estimate the surface temperature of the roller from the sensed temperature. Moreover, since the output of the thermistor spaced apart from the heat roller is susceptible to the ambient temperature, the surface temperature to which the heat roller is controlled differs from the time just after the warm-up of the equipment to the time after a continuous operation.